1. Field of the Invention
The present invention refers to a sensor for measuring forces and/or moments, to a method for measuring forces and/or moments, as well as to a system for measuring forces and/or moments on an object, for example on a device for minimally invasive surgery.
2. Description of the Prior Art
It is known, for example, to use fiber Bragg gratings (FBG) with sensors for measuring forces and/or moments. Here, the degree of expansion of the fiber Bragg grating, and thus the degree of expansion of the object on which the sensor is arranged, is determined from the light reflected by the fiber Bragg grating. The degree of expansion can then be used to determine a force and/or a moment acting on the object.
With such measurements, however, the measuring result strongly depends on temperature, since the object, and thus the fiber Bragg grating, expands in dependence on temperature and as a function of the force or the moment acting, and also the reflection behavior of the fiber Bragg grating changes due to the thermo-optical coefficients of the fiber in which the fiber Bragg grating is provided. The measured wavelength of the fiber Bragg grating is not suited for an ex post determination of which proportion results from temperature variation and which proportion results from variation by expansion.
According to a known possibility for compensation, a sensor is used to measure the temperature at a point of the object, e.g. by means of another fiber Bragg grating, and to mathematically compensate for the temperature variation proportion of the sensor signal. However, this requires that the additional fiber Bragg grating is arranged in a manner isolated against expansion. It is another drawback of this invention that it supposes a uniform temperature across the entire sensor body. If, for example, such a sensor is used on a minimally invasive surgical instrument, one cannot assume a defined temperature of the entire sensor since, for example, flushing solutions or contact with organs may lead to an inhomogeneous temperature distribution across the instrument. On the other hand, using a plurality of measuring points for temperature measurement is often impractical or impossible for reasons of space; further, it is also possible that the entire sensor characteristic is changed by additional temperature sensors.
The applicant has developed a sensor for measuring forces and/or moments, wherein a fiber Bragg grating is provided in a single-mode fiber. Moreover, the material of the single-mode fiber is doped with a fluorescent material. The fiber Bragg grating is illuminated with broadband light and the peak wavelength of the reflected light is detected. The main emission wavelength of the fluorescent radiation differs from the wavelength range of the peak wavelength of the reflected light of the fiber Bragg grating. It is thus possible to determine the expansion of the object, without temperature influences, from the fluorescence lifetime of the fluorescent radiation and the peak wavelength of the reflected light of the fiber Bragg grating.
However, such sensors must have a certain length since the number of atoms excited to fluorescence is limited due to the very small diameter of the single-mode fiber core and therefore only very little fluorescent radiation is reflected. The amount of light of the fluorescent radiation can be so small that the signal-to-noise ratio does not allow for a sufficiently accurate compensation. Often, the marginal conditions do not allow for an elongation of the measuring site and thus for an enlarging of the region in which the fluorescent material is provided. On the other hand, a heavier doping of the glass fiber material is often impossible as well, since the properties of the glass mixture could be modified by the doping and, in addition thereto, the doping atoms in some fluorescent doping materials influence each other strongly when the concentration is high, whereby the measurement could be compromised.